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E. R. Watts & Son. Ltd., London MK II Balloon Theodolite

Image of Watts MKII Pibal Theodolite, showing eyepiece, micrometers, gunsite
image of  rear side of Watts MKII showing objective telescope
This is the 2nd version of an instrument in a series of theodolites that was the standard in the meteorological services of the British Commonwealth.  They were designed at the Meteorological Office in collaboration with E. R. Watts & Son LTD.  5 different models were produced Mk I through MK V.  The models were produced as successive improvements over earlier models.   

The Watts Mark II had mechanical improvements on the Mark I instrument.  A telescope gear that was directly driven by the micrometer tangent screw was added in this model as opposed to driving the circle and connecting this through a bevel gear to the telescope.  In comparison to the Watts MK I this instrument, and later instruments have an different shaped housing to encase a large telescope drive gear.  The MKII instrument had color filters that fit over the objective rather than later models where the filters fit over the eyepiece.  This instrument has a variable focus objective and the eyepiece focus lacks markings that would have facilitated diopter referenced adjustment.  Its battery box is built into the instrument like the Mark I instrument. 

The second image shows the rear readouts of the circles.  The micrometer drums can also be read from the rear of the instrument allowing a second observer to log the instruments position while the primary observer can keep the theodolite trained on the balloon.

In common with other land based balloon theodolites it has a telescope with a bent optical axis.  A secondary wide-angle telescope using the same eyepiece selectable with a mirror.     Follow this link for a more detailed description and Cutaway Illustration of a Watts MK I Balloon Theodolite.   

Like the vast majority of balloon theodolites the image in inverted when viewed in a Watts theodolite.  All Watts Balloon theodolites feature micrometer drums that drive the circles with tangent screws.  The tangent screws and micrometers can be disengaged for rapid motion and positively reengaged for accurate tracking. This Mark II was produced in 1943.

These theodolites incorporate a lined gradicule to support the tail method of altitude determination.  The spacing of the gradicule lines was changed during the evolution of the theodolite series.   Slide rules manufactured as early as 1941 (and possibly much sooner) have notes explaining two procedures for calculations based on two gradicule line spacings.  A Met. Office Instruction sheet dating May 1927 contains instructions for changing the length of the tail to match the line spacing of the gradicule, but I believe that this is to allow for small variations in the gradicule not, a change in manufacturing.  

 
Drive Type Positively Engaging Tangent Screw Compass No
Display Type Micrometer .1 degree Levels 1 tribrach base plate, 1 Inst, body bubble tube
Main Telescope 35 mm 16x 2deg. Mounting Tribrach
Finder Telescope 12.5mm 4.5x 8deg.
single eyepiece
Illumination 2, 3 Volt lamps, scales
reticule
Gun Site Yes 1, on finder telescope Weight (theodolite only) 8.4 kg